Conventionally, a headrest is generally installed in a vehicle seat for an automotive to improve safety of an occupant from an accident such as collisions between vehicles. This headrest possesses a height adjusting mechanism so that an arbitrary point can adjust height location by a physical constitution and preference of an occupant. Generally, two headrest stays are adhered to the headrest. The headrest stays are inserted into two headrest bushing anchored at the side of the seat back respectively and then supported.
In the headrest, a head impact force of an occupant seating on a seat becomes a load in the case of a collision, thus the force that is going to depress the headrest is exerted. Due to the force exerted, the headrest moves to the lower portion, an occupant's head cannot be supported, and the security of safety for the occupant is difficult because the headrest is not in condition that the occupant's head is not completely covered. Therefore, a vehicle seat with a height adjusting mechanism is provided in one of headrest bushings in support of the headrest stays (Japanese Patent Laid-Open No. 1992-193109 Official Gazette). The height adjusting mechanism permits a movement to the upper portion of the headrest, but the movement to the lower portion comes to be locked, and a button to release the lock is provided therefor.
FIG. 10 depicts an exploded perspective view showing the vehicle seat. The numeral 1 of FIG. 10 depicts a seatback, 2 of FIG. 10 depicts a headrest, and 3a and 3b are designated as headrest stays in support of a headrest 2. The numerals 6a and 6b of FIG. 10 are designated as headrest bushings into which a headrest stay 3 is inserted and is supported. In the headrest bushing 6a, a height adjusting mechanism 7, which allows the movement to the upper portion of the headrest stay 3a and prevent the movement to the lower portion, is installed freely in advance or retreat in a perpendicular direction to the headrest stay 3. The height adjusting mechanism 7 is formed by an abutting lock plate 8 in contact with the headrest stay 3a, and a spring 12 (not shown). An outside of the height adjusting mechanism 7 is covered by a housing 13, a button 8b of the abutting lock plate 8 is only exposed to the outside. The headrest stay 3a is provided with height adjusting notch 4 in a predetermined space (hereinafter referred to as a “height adjusting notch”). The height adjusting notch 4 is formed by a horizontal surface 4a toward the center of the headrest stay 3a, and an inclined surface 4b to increase the diameter of slowly downward from a deepest portion of the horizontal surface 4a. In the height adjusting mechanism 7, by pulling the headrest 2 upward an abutment 8a in contact with the headrest stay 3a of the lock plate 8 is disengaged from the height adjusting notch 4 against an elastic force of a spring 12 by action of the incline surface 4b of the height adjusting notch 4. The headrest 3a is movable upward, and the headrest 2 can be adjusted in its height. On the contrary, the lock plate 8 is not disengaged from the height adjusting notch 4 in engagement with a horizontal surface 4a of the height adjusting notch 4 when the headrest 2 is pushed downward. This can prevent from moving to the lower portion of the headrest 2, however, the button 8b of the lock plate 8 is disengaged from the height adjusting notch 4 by pushing against the spring 12, thereby moving to the lower portion of the headrest stay 3a. 
A falling-off preventive mechanism is arranged to the headrest bushing in a free state for preventing the headrest stay from falling-off, the falling-off preventive mechanism is also provided in the inside of the seatback (U. S. patent application publication US2005/200185).
However, the headrest stay is inserted into the two headrest bushings, one of which is in a free state without having any of mechanisms according to the vehicle seat in Japanese Patent Laid-Open No. 1992-193109 Official Gazette. Therefore, the headrest is disengaged from the seatback easily when an upward force acts on the headrest (for example, it may be contemplated in the case that an occupant pulls the headrest out).
Consequently, in the other headrest bushing with the height adjusting mechanism there is provided with a vehicle seat comprising: a locking mechanism for stopping the movement to the upper portion of the headrest stay and for preventing disengagement of the headrest, and a mechanism having a locking/unlocking mechanism (hereinafter referred as to a “falling-off preventive mechanism”).
FIG. 11 shows a main exploded perspective view of a vehicle seat having the headrest comprising the falling-off preventive mechanism. All of the components which occur identically in FIG. 10 have the same reference numerals respectively, a description on the components is omitted.
The numeral 5 of FIG. 11 is designated as a notch for preventing the headrest 2 from falling-off (hereinafter referred as to a “falling-off preventive notch”), the numeral 9 is the falling-off preventive mechanism, and the numeral 10b is an operating button for unlocking the falling-off preventive mechanism 9. The headrest bushing 6b is embedded in the inside of the seatback 1, and the falling-off preventive mechanism 9 has a construction to be installed in the upper portion in the form protruded outside from the seatback 1. The falling-off preventive mechanism 9 is installed freely in advance or retreat in the perpendicular form to the headrest stay 3b. The headrest stay 3b is inserted through the falling-off preventive mechanism 9. The falling-off preventive mechanism 9 comprises the lock plate 10 and the spring 12 (not shown). The lock plate 10 comprises a button 10b to unlock an abutment (not shown) in contact with the headrest stay 3b. The falling-off preventive notch 5 is formed by a horizontal surface 5a, and an inclined surface 5b to increase the diameter slowly upward from the deepest portion of the horizontal surface 5a. When the lock plate 10 reaches the position of the falling-off preventive notch 5, the abutment (not shown) of the lock plate 10 engages the horizontal surface 5a of the falling-off preventive notch 5. This allows to lock the movement to the upper portion of the headrest stay 3b. When the locked movement is unlocked, by pushing the outside exposed button 10b of a housing 13 the abutment (not shown) disengages the horizontal surface 5a of the falling-off preventive notch 5, and the locked headrest stay 3b is unlocked.
However, in the headrest busing 3b as mentioned above when the falling-off preventive mechanism 9 unlocks the locked headrest stay 3b, the button 10b for unlocking the locked headrest stay 3b is exposed to the outside of the housing to accommodate the falling-off preventive mechanism 9. Accordingly, even supposing that the occupant merely touches, if the button 10b is pushed, the locked movement to the upper portion of the headrest 2 may be sufficiently unlocked. Even though the lock may be unlocked with such a mere touch, the headrest may be disengaged when the occupant does not intend, and a dangerous situation also may occur.
The present invention is to provide the headrest bushing having the headrest 2 not dislocated from the seatback 1 and the vehicle seat therewith, if the invention is performed, not by an intentional action forcing the headrest 2 to be disengaged from the seatback 1.